Reduction of monomer content and stabilization of polyaziridines

ABSTRACT

The invention relates to a process for reducing the monomeric aziridine content in a polyaziridine forming reaction mixture by adding to the polyaziridine forming reaction mixture an excess of a carbodiimide scavenger wherein the excess is based on the equivalent ratio of scavenger to monomeric aziridine, and to a product obtained by this process and to a coating composition containing the product obtained by the process.

BACKGROUND OF THE INVENTION

The invention relates to a process for reducing monomeric aziridines ina polyaziridine reaction by adding carbodiimides as a scavenger, theproducts obtained by that process and coating compositions prepared fromthese products.

Polyfunctional aziridines have been shown to be useful as crosslinkingagents in various types of waterborne and solventborne coating systemssuch as carboxylated acrylics, vinyl-acetate, carboxylated urethanessuch as polyurethane dispersions (PUDs), styrene acrylics or mixturesthereof.

One important subclass of commercially available polyfunctionalaziridines includes reaction products of ethylene imine (EI, aziridine)or propylene imine (PI, methyl aziridine) with trimethylol propanetriacrylate (described for example in U.S. Pat. No. 2,596,299 toBastian). Other commercially important polyfunctional aziridines can beprepared from ethylene imine or propylene imine and pentaerythritoltriacrylate. Other polyfunctional aziridines are based on alkoxylatedpolyols.

Another method of preparing polyfunctional aziridines includes thetransesterification of methyl (1-aziridinyl)propionates with polyolscatalyzed with tertiary amines (as described in DE 2334656 toMiksovsky), whereby the methyl (1-aziridinyl)propionates are preparedfrom monomeric aziridines.

Ethylene imine and propylene imine are volatile low molecular weighttoxic compounds which are undesired residuals in the processes to formpolyfunctional aziridines. The residual monomeric aziridine compound hasto be removed sometimes tediously by elaborate distillation methods orlengthy aging processes. This is especially true, if the reactiontemperature is low, which is sometimes necessary to avoid discolorationor viscosity problems. Sometimes the residue can exceed 1000 ppm incrude reaction mixtures. It is very desirable to reduce this amount fora commercial product below 500 ppm and even more preferred to reduce itbelow 100 ppm or ultimately below 50 ppm.

To drive the Michael-type addition reaction of the aziridine and theacrylate to completion, it is possible to use an excess of aziridine. Adisadvantage of this approach is the amount of aziridine to be removedfrom the reaction product under vacuum. With a simple distillationcolumn and a vacuum of about 50 mm Hg it is usually not possible toremove the aziridine level to below 300 ppm on a commercial time scale,for example in a day. To achieve an aziridine level below that, it isnecessary to use refined equipment, like an expensive falling-film orwiping-film evaporator, or longer distillation times which iseconomically unfavorable. Another method to drive the Michael additionuses an excess of acrylate. The residual aziridine levels can thereby bereduced to less than 50 ppm, which sometimes require considerable agingtimes up to several months, which again is economically unfavorable.

It is an object of the present invention to provide an easy process formaking polyfunctional aziridines without advanced and expensivedistillation or purification steps. It is another object of theinvention to produce polyfunctional polyaziridines that arerheologically stable and are low in color. It is another object of theinvention to develop a process that results in low monomeric aziridinewithout incurring elaborate vacuum steps or long batch or aging times

The present invention uses a scavenger. The reactions betweenelectrophiles and aziridines have been described in detail in theliterature. Copending application Ser. No. 10/137,805 describes anisocyanate as a scavenger. One example of a suitable electrophile is acarbodiimide.

The reaction between carbodiimides and aziridines is known. Baeg et al.disclose a regiospecific palladium-catalyzed cycloaddition ofN-substituted aziridines and carbodiimides in the J. Org. Chem. (1992),57(1), 157-62.

None of the above disclosures however describes the present invention.

SUMMARY OF THE INVENTION

The invention relates to a process for reducing monomeric aziridines ina polyaziridine forming reaction mixture by adding to the polyaziridineforming reaction mixture an excess of a carbodiimide scavenger, whereinthe excess is based on the equivalent ratio of scavenger to monomericaziridine.

The invention also relates to the product obtained by this process and acoating composition containing this product.

DETAILED DESCRIPTION OF THE INVENTION

Various aziridines and substituted aziridines can be used to formpolyfunctional aziridines. The suitable aziridines are well known in theart and generally correspond to the formula

where R₁, R₂, R₃, and R₄ independently represent hydrogen; alkyl with upto about 20 carbon atoms, preferably methyl, ethyl, or propyl; aryl,preferably phenyl; alkaryl, preferably tolyl or xylyl; or aralkyl,preferably benzyl or phenethyl.

The groups R₁-R₄ may represent substituted radicals wherein thesubstituents include cyano, halo, amino, hydroxy, alkoxy, carbalkoxy,and nitrile. Suitable examples of substituted groups R₁, R₂, R₃, and R₄thus include cyanoalkyl, haloalkyl, aminoalkyl, hydroxyalkylalkoxyalkyl, carbalkoxyalkyl, and similar substituted derivatives ofaryl, alkaryl and aralkyl groups.

Specific examples of suitable aziridines include ethylenimine(aziridine), 1,2-propylenimine (2-methyl aziridine), 2-ethyl aziridine,1,2-dodecylenimine (2-decyl aziridine), 1,1-dimethyl ethylenimine(2,2-dimethyl aziridine), phenyl ethylenimine (2-phenyl aziridine),tolyl ethylenimine (2-(4-methylphenyl) aziridine), benzyl ethylenimine(2-phenylmethyl aziridine), 1,2-diphenyl ethylenimine (2,3-diphenylaziridine), hydroxyethyl ethylenimine (2-(2-hydroxyethyl) aziridine),aminoethyl ethylenimine (2-(2-aminoethyl) aziridine), 3-chloropropylethylenimine (2-(3-chloropropyl) aziridine), p-chlorophenyl ethylenimine(2-(4-chlorophenyl) aziridine), methoxyethyl ethylenimine(2-(2-methoxy-ethyl) aziridine), dodecyl aziridinyl formate (dodecyl1-aziridinyl carboxylate), carbethoxyethyl ethylenimine(2-(2-carbethoxyethyl) aziridine).

Because of their availability and because they have been found to beamong the most effective, the preferred aziridines are ethylenimine,1,2-propylenimine and 2-ethylaziridine.

The suitable aziridines are usually reacted with acrylates.

Preferred acrylates are polyacrylates having a functionality f≧2, whichcan be synthesized e.g. by an esterification reaction between a polyoland acrylic acid. However, other methods are also possible to synthesizethose polyfunctional acrylates. Examples of polyols used in this kind ofpolyacrylate synthesis include neopentyl glycol,2,2′-bis(p-hydroxy-phenyl)propane (bis-phenol A),bis(p-hydroxyphenyl)methane (bis-phenol F), glycerol,trimethylolpropane, pentaerythritol and others. It is also possible touse diols commonly used in polyester synthesis. Examples of these diolsinclude ethylene and propylene glycol, butandiol, hexanediol and others.

It is also possible but less preferred to react the monomeric aziridinewith monoacylates e.g. esters of acrylic and methacrylic acid andsubsequently perform an optionally base catalyzed transesterificationreaction.

The reaction temperature in the Michael-addition between the acrylateand the aziridine is above the melting point of the components and below100° C. Reactions at room temperature will usually work well. It ispreferred to react between 0 and 60° C., more preferred between 25 and50° C. Theoretically higher temperatures can be applied, but are notpreferred. It is also possible to run the reaction under pressure withor without elevated temperatures, if suitable equipment is available,which is less preferred. Typically the monomeric aziridine is added tothe acrylate over a period of time to control the exothermic reaction.The reaction is then performed in the above mentioned temperature range.Typically the reaction time is less than 24 hours, but will vary withbatch size. It is also possible to add the acrylate to the monomericaziridine, however this is less preferred. In either case, at the end ofthe reaction the monomeric aziridine level is usually less than 1000ppm, for example in the range between 100 ppm and 1000 ppm. In caseswhere the monomeric aziridine level is significantly higher it ispossible to apply a weak vacuum (around 50 mm Hg) and use a distillationcolumn to reduce the monomeric aziridine level into the above mentionedrange. However, to further reduce the monomeric aziridine contentsophisticated vacuum equipment or very long application times arenecessary.

To reduce the level of monomeric aziridine to below 50 ppm according toone embodiment of the invention an excess of a suitable carbodiimidescavenger is added to the reaction mixture, wherein the excess is basedon the equivalent ratio of scavenger to monomeric aziridine. Theinvention uses an equivalent ratio of scavenger to residual monomericaziridine of 1:1 to 15:1, preferably 1:1 to 14:1, more preferably 1.7:1to 3:1. In one embodiment of the invention the ratio is as low as 1:1 to1.01:1. If the residual amount in the polyaziridine forming reactionmixture is between 200 and 1000 ppm, usually 0.01 to 2% by wt of thescavenger, based on the reaction mixture is sufficient. Preferredamounts of scavenger are between 0.05 and 1.5% by wt., more preferredbetween 0.1 and 1.0% by wt., and most preferred between 0.15 and 0.6% bywt., based on the weight of the reaction mixture.

The monomeric aziridine-scavenger-reaction can be performed at ambienttemperature. Since the reaction is exothermic a means for constantlycooling the reaction can be optionally employed.

The scavenger is generally added slowly into the reactor containing thepolyaziridine forming reaction mixture. The reactor is usually equippedwith a condenser, a stirring means and a temperature measuring device.If the acrylate in this reaction is a polyacrylate then the reaction isfinished; however, if the acrylate is a monoacrylate atransesterification reaction can be performed afterwards. In this lesspreferred case it is possible to perform the Michael-reaction first,then the transesterification reaction and finally the scavengingreaction.

Scavengers are compounds that react fast with the NH-functionality ofaziridines. Several classes of compounds fit this description. In thecontext of the invention it is also desirable that the resultingmixtures of the polyaziridine and the reaction product of the aziridineand the scavenger are useful for coatings applications in that they haveappropriate properties including color stability and rheologicalstability. Additionally, suitable scavengers in the context of thepresent invention do not release acidic by-products. Examples ofsuitable scavengers include carbodiimides.

Suitable carbodiimides include symmetric and unsymmetric carbodiimidesrepresented by formulaR′—N═C═N—R″wherein

-   -   R′ and R″ independently represent alkyl and cycloalkyl with up        to about 20 carbon atoms, aryl, alkaryl, or aralkyl.        Preferred residues R′ and R″ include methyl, ethyl, propyl,        cyclohexyl, phenyl, tolyl, xylyl, benzyl or phenethyl. Examples        include N,N-dicyclohexylcarbodiimide (DCC),        1,3-diisopropylcarbodiimide, 1,3-di-tertbutylcarbodiimide,        1,3-di-sec-butylcarbodiimide, N,N′-di-o-tolylcarbodiimide,        bis-(2,6-diisopropylphenyl)-carbodiimide, or        1-ethyl-3-(3dimethylaminopropyl)-carbodiimide.

It is of course also possible to add mixtures of scavengers (e.g.different carbodiimides). Some of the scavengers can also be mixtures ofdifferent stereo- and regioisomers.

It can be useful to add a stabilizer to the reaction mixture. It ispreferred to use tertiary amines, more preferred the use of aliphatictertiary amines and especially preferred is the use oftetramethylethylene diamine (TMEDA). Other additives e.g. antioxidants,rheology modifier, light stabilizer among others can be added asnecessary for the final application, if they are not nucleophilic oracidic in nature.

It is possible to add the stabilizer to the reaction mixture before,during or after the reaction.

The scavenger can be added with or preferred without solvent or solventmixtures, however preferred solvents are non-nucleophilic and non-acidicin nature. In general, it is preferred to add the scavenger or themixture of scavengers neat without solvent.

Catalysts that enhance the reactivity of the scavenger are usually notnecessary. Preferred catalysts are non-nucleophilic and non-acidic innature.

The products prepared by the present invention can be used in all knownpolyaziridine applications, including use as cross-linkers, in adhesiveapplications, in coating compositions and in inks and printingcompositions, in the field of photography, thermal and electrostaticimaging, fiber and fabric treatment and other uses.

The invention is further illustrated but is not intended to be limitedby the following examples in which all parts and percentages are byweight unless otherwise specified.

EXAMPLES

Production of Polyaziridine (I)

4800 lbs trimethylolpropane triacrylate were charged to a reactor and 17lbs 4-methoxyphenol were added as an inhibitor against free radicalpolymerization. 2 lbs triethylamine were added to the reactor tomaintain basic conditions.

An excess of 10% molar excess of propylene imine (2-methyl-aziridine)was then added slowly over 10-12 hours, while keeping reactiontemperature at 35-40° C.

After all of the propylene imine was added, the temperature wasmaintained around 37.5° C. for about 9 hours. Some propylene imine wasremoved by distillation at around 50 mm Hg for 7 hours.

The polyaziridine product contained between 200 ppm and 300 ppm ofresidual propylene imine.

The examples below were compared by mixing at room temperature 0.5 gramsof the scavenger (such as N,N-dicyclohexylcarbodiimide (DCC) inexample 1) with 100 grams of polyaziridine I. The resulting productswere clear, slightly viscous liquids.

This dramatically lowered the PI content from about 214 ppm to less than50 ppm. The modified polyaziridine was unaffected in terms of physicalproperties and application properties and provided a high viscosity andhigh color stability.

Initial Final Scavenger PI PI Weight (ppm) (ppm) Example Scavenger GramsLC-MS LC-MS Appearance 1 DCC 0.50 214 30 clear Comp 1 Benzoyl 0.50 268 4 hazy chloride Comp 2 Phthalic 0.50 268 n.d. did not anyhyridedissolve Unmodified DCC-modified Storage at polyaziridine (I)polyaziridine (I) (EX. 1) 50° C. Color Viscosity Color Viscosity 0 weeks 61  434  78 530 1 week 102  892  94 534 2 weeks 115 1170 104 583 3weeks 140 1740 120 587

The examples also show that not all possible scavengers result in resinsthat are commercially usable, but that the scavengers according to theinvention have excellent results.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

1. A process for reducing the monomeric aziridine content in apolyaziridine forming reaction mixture comprising adding to thepolyaziridine forming reaction mixture an excess of a carbodiimidescavenger wherein the excess is based on the equivalent ratio ofcarbodiimide scavenger to monomeric aziridine; wherein the polyaziridineforming reaction mixture comprises monomeric aziridine, acrylates, andpolyaziridines; and wherein the carbodiimide scavenger comprisessymmetric and unsymmetric carbodiimides represented by the formulaR′—N═C═N—R″ wherein R′ and R″ independently represent alkyl andcycloalkyl with up to about 20 carbon atoms, aryl, alkaryl, or aralkyl.2. The process of claim 1 wherein the equivalent ratio of carbodiimidescavenger to monomeric aziridine is between 15:1 to 1.01:1.
 3. Theprocess of claim 1 wherein the equivalent ratio of scavenger tomonomeric aziridine is between 3:1 and 1.01:1.
 4. The process of claim 1wherein the polyaziridine forming reaction mixture has a content ofmonomeric aziridine between 100 ppm and 1000 ppm.
 5. The process ofclaim 4 comprising adding between 0.05 and 1.5% by weight of thecarbodiimide scavenger based on the polyaziridine forming reactionmixture.
 6. The process of claim 4 comprising adding between 0.1 and1.0% by weight of the scavenger.
 7. The process of claim 4 comprisingadding between 0.15 and 0.6% by weight of the scavenger.
 8. The processof claim 1 comprising reacting an acrylate component and an aziridinecomponent to form the polyaziridine forming reaction mixture.
 9. Theprocess of claim 8 comprising reacting an acrylate component and anaziridine component before adding the scavenger.
 10. The process ofclaim 8 comprising adding a stabilizer to the reaction mixture before,during or after the reaction.
 11. The process of claim 10 wherein thestabilizer comprises a tertiary amine.
 12. The process of claim 10wherein the stabilizer is tetramethylethylene diamine.
 13. The processof claim 8 comprising reacting the components at a temperature above themelting point of the aziridine compound and below 100° C.
 14. Theprocess of claim 1 comprising adding between 0.05 and 1.5% by weight ofthe carbodiimide scavenger based on the polyaziridine forming reactionmixture.
 15. The process of claim 1 wherein the scavenger comprises asymmetric carbodiimide.
 16. The process of claim 1 wherein the scavengercomprises an unsymmetric carbodiimide.
 17. The process of claim 1wherein the scavenger is N,N-dicyclohexylcarbodiimide (DCC).